A microstructure-regulated wind power spindle steel, its heat treatment preparation method and application

Abstract

The present invention relates to the field of metallurgical technology, and specifically relates to a wind power spindle steel with microstructure regulation, its heat treatment preparation method and application. The wind power spindle steel is prepared by a process of smelting - LF refining - VD refining - continuous casting and rolling - profiling forging - performance heat treatment. Its elemental composition includes: C, Si, Mn, Nb, Cr, Mo, Al, V, Ti, N, P, S, Fe and inevitable impurities. In this wind power spindle steel, the proportion of small-angle grain boundaries less than 10° is greater than 65%; there are micro-nano sized (Nb, V)N precipitation phases at the grain boundaries. The wind power spindle steel has a tensile strength exceeding 900 MPa and an elongation rate exceeding 14%, and has good properties.

Description

Technical Field

[0001] The present invention relates to the field of metallurgical technology, and particularly to a wind power spindle steel with microstructure regulation, its heat treatment preparation method and application. Background Art

[0002] With the growth of global energy demand and the increasingly serious environmental problems, finding clean and renewable energy has become the focus of common concern around the world. As an important clean energy, wind power generation can not only reduce the dependence on fossil fuels and lower greenhouse gas emissions, but also has broad development prospects. The progress of wind power generation technology, especially the research and development of large-scale wind turbines, is of great significance for promoting the transformation of the energy structure and achieving sustainable development goals.

[0003] In a wind power generation unit, the wind power spindle occupies an indispensable position due to its core supporting role and key transmission function. As a key component connecting the wind turbine rotor to the gearbox or generator, the wind power spindle undertakes the important task of smoothly transmitting the kinetic energy captured by the wind turbine rotor to subsequent components, and finally realizes the conversion of mechanical energy into electrical energy. Its stable and reliable working state directly determines the efficiency and stability of the entire wind power generation system. Therefore, the design, manufacture and maintenance of the wind power spindle have become one of the important factors to ensure the efficient operation of wind power generation projects.

[0004] The wind power spindle can be regarded as the backbone of a wind turbine and is one of the core elements determining its performance. Given that wind turbines are mostly installed in areas with relatively harsh environmental conditions, such as offshore platforms or high-altitude land, the wind power spindle must be able to adapt to and resist various complex working conditions, including but not limited to adverse factors such as extreme temperature fluctuations, high humidity, salt spray corrosion, and unstable wind force changes. In addition, with the rapid development of wind power generation technology, the single-unit capacity of wind turbines has continuously climbed to higher megawatt levels. This not only indicates a significant improvement in the power generation capacity of a single wind turbine, but also poses more stringent requirements on the wind power spindle: larger size, heavier load, stronger durability. Especially under high-intensity wind conditions, the wind power spindle needs to withstand huge rotational stresses, which puts extremely high requirements on its structural strength and material properties.

[0005] To address these challenges, alloy steel with relatively good comprehensive mechanical properties is often selected as the material for wind turbine main shafts. However, the properties of alloys, such as strength, are jointly affected by many factors, including element composition, process parameters, metallographic microstructure, and so on. Among them, grain boundary energy is an important factor affecting the mechanical properties of alloy steel, and grain boundary energy is significantly affected by the grain boundary angle. Generally, adjacent grains with a phase difference of less than 10° have a grain boundary called a small-angle grain boundary; adjacent grains with a phase difference greater than 10° have a grain boundary called a large-angle grain boundary. Increasing the proportion of fine particles in the small-angle grain boundary is beneficial to reducing the grain boundary energy of the alloy and improving the alloy properties. Patent document CN114381641A provides an Al-Mg-Zn-RE alloy with a high density of small-angle grain boundaries, but its strength is about 300 MPa, which is relatively low. CN114686776A provides a super-thick steel plate with small-angle grain boundaries with a phase difference of 3° to 15° and large-angle grain boundaries with a phase difference greater than 15° distributed in the grains, but its strength is lower than 700 MPa, which is relatively low. Summary of the Invention

[0006] Aiming at the above problems, the purpose of the present invention is to provide a wind turbine main shaft steel with microstructure regulation, its heat treatment preparation method and application, so as to improve the strength, elongation rate, etc. of the wind turbine main shaft steel. Therefore, the technical solution of the present invention is as follows:

[0007] In the first aspect, the present invention provides a wind turbine main shaft steel. The element composition of the wind turbine main shaft steel is as follows by mass percentage: C: 0.18 - 0.32%; Si: 0.21 - 0.38%; Mn: 1.11 - 1.34%; Nb: 0.41 - 0.49%; Cr: 1.82 - 1.96%; Mo: 0.36 - 0.41%; Al: ≤0.01%; V: 0.36 - 0.45%; Ti: 0.04 - 0.08%; N: 0.010 - 0.014%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and unavoidable impurities; the proportion of small-angle grain boundaries less than 10° in the wind turbine main shaft steel is greater than 65%.

[0008] Preferably, the proportion of small-angle grain boundaries less than 10° in the wind turbine main shaft steel reaches 67% - 69%. The proportion of small-angle grain boundaries can be obtained by irradiating the crystal surface of the sample with X-ray diffraction to obtain the grain orientation information of the material, and then statistically obtaining it.

[0009] Preferably, there are micro-nano-sized (Nb, V)N precipitation phases at the grain boundaries of the wind turbine main shaft steel. The micro-nano-sized precipitation phases can be obtained by routinely preparing the metal microstructure for microscopic observation and detection.

[0010] In the second aspect, the present invention provides a preparation method for the aforementioned wind turbine main shaft steel, including the following steps:

[0011] S1 Smelting:

[0012] S11 Add raw materials to a furnace for smelting to produce molten iron.

[0013] S12 Blow oxygen into the molten iron obtained in step S11, and add quicklime for slag-making treatment.

[0014] S13 Remove impurities by slag blocking to obtain smelted molten steel. The elemental composition of the smelted molten steel is by mass percentage: C: 0.04 - 0.06%; Si: 0.07 - 0.13%; Mn: 0.33 - 0.55%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and inevitable impurities.

[0015] S2 LF refining:

[0016] S21 Transfer the smelted molten steel obtained in step S13 to an LF refining furnace.

[0017] S22 Heat up the molten steel in step S21, and add ferrovanadium and ferroniobium master alloys to adjust the chemical composition.

[0018] S23 Blow argon for stirring, and then feed aluminum wire for deoxidation.

[0019] S24 Supplement quicklime and fluorite, make slag, and obtain refined molten steel. The elemental composition of the refined molten steel is by mass percentage: C: 0.18 - 0.32%; Si: 0.21 - 0.38%; Mn: 1.11 - 1.34%; Nb: 0.41 - 0.49%; Cr: 1.82 - 1.96%; Mo: 0.36 - 0.41%; Al: ≤0.01%; V: 0.36 - 0.45%; Ti: 0.04 - 0.08%; N: 0.001 - 0.003%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and inevitable impurities.

[0020] S3 VD refining:

[0021] S31 Transfer the refined molten steel obtained in step S24 to a VD vacuum furnace and conduct vacuum pumping treatment.

[0022] S32 Blow nitrogen from the bottom to break the vacuum and increase the nitrogen content in the molten steel.

[0023] S33 Feed in the calcium wire to further purify the molten steel, obtaining high-nitrogen cast molten steel; the elemental composition of the high-nitrogen cast molten steel is by mass percentage: C: 0.18~0.32%; Si: 0.21~0.38%; Mn: 1.11~1.34%; Nb: 0.41~0.49%; Cr: 1.82~1.96%; Mo: 0.36~0.41%; Al: ≤0.01%; V: 0.36~0.45%; Ti: 0.04~0.08%; N: 0.010~0.014%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and unavoidable impurities;

[0024] S4 Continuous casting and rolling:

[0025] S41 Pour the high-nitrogen cast molten steel obtained in step S33 into a mold for continuous casting, obtaining a cast round billet;

[0026] S42 Perform blooming on the cast round billet obtained in step S41, and then perform rolling. The rolling undergoes 2 deformations each of cross rolling and longitudinal rolling. The cast round billet is then hot-delivered to an ingot at 900°C~950°C;

[0027] S5 Profiled forging:

[0028] S51 Heat the hot-delivered ingot to 1260°C~1280°C and hold for 5h~6h to ensure uniform heating; then perform 2 times of repeated drawing-ups and upsetting in sequence, obtaining a repeatedly drawn-up and upset ingot;

[0029] S52 Reheat the repeatedly drawn-up and upset ingot obtained in step S51 and then perform punching;

[0030] S53 Reheat the forging after punching in step S52 and then perform drawing-up and rolling of the shaft body to complete the shaft body forming, obtaining rolled round steel;

[0031] S6 Performance heat treatment:

[0032] S61 Place the rolled round steel obtained in step S53 into a slow-cooling pit for multi-stage slow cooling treatment, obtaining multi-stage slow-cooled rolled round steel;

[0033] S62 Take out the multi-stage slow-cooled rolled round steel obtained in step S61, and after controlling the cooling to room temperature, obtain wind power spindle steel.

[0034] Preferably, the smelting temperature in step S1 is 1600°C~1650°C, and the smelting time is based on the C content reaching the standard.

[0035] Preferably, in step S2, the LF refining temperature is 1720°C to 1770°C, the argon flow rate is 200 NL / min to 300 NL / min, the amount of aluminum wire fed is 4 kg / ton to 5 kg / ton, the amount of quicklime added is 8.6 kg / ton to 8.9 kg / ton, the amount of fluorite added is 1.8 kg / ton to 2.0 kg / ton, and the refining time is calculated as 17 min / ton to 19 min / ton.

[0036] Preferably, in step S3, the vacuum degree of VD refining is ≤55 Pa, the refining temperature is 1780°C to 1800°C, the nitrogen flow rate is 200 NL / min to 300 NL / min, the amount of calcium wire fed is 0.1 kg / ton to 0.12 kg / ton, and the refining time is calculated as 16 min / ton to 19 min / ton.

[0037] Preferably, in step S4, the first rolling temperature of cross rolling is 1150°C to 1200°C, and the reduction ratio is 15% to 17%; the first rolling temperature of longitudinal rolling is 1100°C to 1150°C, and the reduction ratio is 12% to 14%; the second rolling temperature of cross rolling is 1050°C to 1100°C, and the reduction ratio is 10% to 12%, and the second rolling temperature of longitudinal rolling is 1000°C to 1050°C, and the reduction ratio is 8% to 10%.

[0038] Preferably, in step S51, the hot delivered ingot is heated to 1260°C to 1280°C at a rate of 50°C / h to 60°C / h and held for 5 h to 6 h, and then the first drawing out and the first upsetting are carried out. The drawing out ratio is 7 to 8.5, the upsetting ratio is 7.3 to 7.6, and the end temperature of the first drawing out - upsetting step is 1000 to 1050°C; then the second drawing out and the second upsetting are carried out. The drawing out ratio is 3.8 to 4.2, the upsetting ratio is 3.8 to 4.1, and the end temperature of the second drawing out - upsetting step is 750°C to 780°C.

[0039] Preferably, the forging after punching obtained in step S52 is reheated to 1180°C to 1200°C for the shaft body drawing out. The shaft body drawing out is carried out in a way that the anvil width ratio is 0.5 to 0.7, the reduction amount is 80 mm to 100 mm, the feeding amount is 0.4 to 0.6 of the anvil width, and a 90° clockwise flipping is adopted.

[0040] Preferably, for the forging obtained in step S52, the temperature range for the rounding treatment is 750°C to 960°C, and the anvil width ratio is 0.7 to 0.8, and the reduction amount is 8 mm to 16 mm.

[0041] Preferably, in the step S6, the rolled round steel is quickly placed into a slow cooling pit, covered and sealed for 5 to 8 hours for slow cooling, then the cover is opened and air is blown in for 15 to 20 minutes to form air cooling. When the temperature drops to 300 - 400 °C, it is covered and sealed again for 8 to 10 hours for slow cooling, and finally taken out and placed in the air for natural cooling to room temperature.

[0042] In a third aspect, the present invention provides the use of the aforementioned wind power spindle steel in the preparation of wind power spindles.

[0043] In a fourth aspect, the present invention provides a wind power spindle made of any one of the aforementioned wind power spindle steels.

[0044] The present invention has the following technical effects or advantages:

[0045] 1. The wind power spindle steel of the present invention and the wind power spindle steel prepared by the method of the present invention have the characteristics of multi-scale microstructure regulation and low grain boundary energy. With the synergistic regulation of the preparation process of smelting - LF refining - VD refining - continuous casting and rolling - profiling forging - performance heat treatment, through the composition control of Nb and V, the supersaturated solid solution is formed by melting control, and the supersaturated solute atoms precipitate and react to form metal nitrides by rolling stress and strain control; controlling temperature and cooling promotes the mutual dissolution reaction of metal nitrides at the grain boundaries to form thermodynamically stable composite nitrides, pinning the grain boundaries, forming a large proportion of small-angle grain boundaries, and reducing the grain boundary energy of the wind power spindle steel. The specific principle is as follows:

[0046] (1) During the preparation process of the wind power spindle steel of the present invention, its composition contains Nb: 0.41 - 0.49%, V: 0.36 - 0.45% and N: 0.010% - 0.014%. During the VD vacuum refining process, the solubility of Nb and V in iron increases significantly with the increase of temperature and is added to the molten iron in the form of master alloy. At the melting temperature, Nb and V can be dissolved in the iron matrix in large amounts, and then during the casting and solidification process, due to rapid cooling, Nb and V form supersaturated solid solutions in the steel.

[0047] (2) During the preparation process of the wind power spindle steel of the present invention, it undergoes cross-rolling and longitudinal rolling with 2 deformations each (the first cross-rolling temperature is 1150°C - 1200°C, the reduction rate is 15% - 17%, the first longitudinal rolling temperature is 1100°C - 1150°C, the reduction rate is 12% - 14%, the second cross-rolling temperature is 1050°C - 1100°C, the reduction rate is 10% - 12%, the second longitudinal rolling temperature is 1000°C - 1050°C, the reduction rate is 8% - 10%). Under the action of gradient compressive stress and strain, a large number of dislocations are generated in the matrix and sub-grain boundaries are formed. Under rolling conditions (1000°C - 1200°C, pressure processing), Nb, V, and N atoms aggregate at dislocations under the stress gradient and precipitate to form NbN and VN. The reaction formulas are as follows: Formation of VN: V + N → VN; Formation of NbN: Nb + N → NbN.

[0048] (3) During the preparation process of the wind power spindle steel of the present invention, during the performance heat treatment process, on the one hand, since the free energy of the solid solution is usually lower than that of a single compound, (Nb,V)N is more stable than VN and NbN existing alone. On the other hand, both VN and NbN are transition metal nitrides with similar crystal structures. The lattice constants of VN and NbN are 4.38 Å and 4.39 Å respectively, and the atomic radii of Nb and V are close (the atomic radius of Nb is about 1.46 Å, and the atomic radius of V is about 1.35 Å). They can replace each other's positions in the lattice at high temperatures (under the temperature condition of 500°C - 700°C), have a mutual reaction, and form the condition for forming a (Nb,V)N solid solution. Thus, finally, a micro-nano (Nb,V)N precipitation phase is formed at the grain boundary, promoting the formation of a high proportion of small-angle grain boundaries and reducing the grain boundary energy of the material.

[0049] 2. Small-angle grain boundaries are usually composed of dislocation arrays. These dislocations form a dense network at the grain boundary, increasing the dislocation density inside the material. The high-density dislocations will hinder the movement of other dislocations, thereby improving the strength of the material. Near the small-angle grain boundary, dislocations are prone to pile-up. Dislocation pile-up can prevent the further slip of dislocations, thereby improving the strength of the material. The formation of small-angle grain boundaries is usually accompanied by finer grains, playing a role in fine-grain strengthening. Finer grains mean a larger grain boundary area. Grain boundaries have a strong hindering effect on the movement of dislocations, thereby improving the strength of the material. Small-angle grain boundaries have lower energy, which makes them easier to move and rearrange during the deformation process. This characteristic helps the material maintain a high strength during deformation.

[0050] 3. Small-angle grain boundaries are usually composed of dislocation arrays, and these dislocations can slip more easily under the action of external forces. This slippage helps to disperse stress, reduce local stress concentration, and thus improve the plasticity of the material. Near small-angle grain boundaries, dislocations can proliferate more easily. The proliferation of dislocations can provide more slip paths, enabling the material to better absorb energy during deformation and enhancing plasticity. Small-angle grain boundaries have less hindrance to dislocations, so they do not strongly impede dislocation movement like large-angle grain boundaries do. This makes it easier for dislocation slip to occur during the deformation of the material, thereby improving plasticity. The presence of small-angle grain boundaries can reduce the total energy of the grain boundaries, making the grain boundaries easier to move and rearrange during deformation. This grain boundary softening effect helps the uniform deformation of the material and reduces the formation and propagation of cracks. Description of the Drawings

[0051] Figure 1 It is the metallographic microstructure diagram and partial enlarged view of the wind power spindle steel in Embodiment 1 of the present invention;

[0052] Figure 2 It is the statistical chart of the proportion of small-angle grain boundaries of the wind power spindle steel in Embodiment 1 of the present invention. Detailed Embodiments

[0053] The present invention will be further described below in conjunction with specific embodiments. It should be understood that the specific embodiments are only used to explain the present invention and are not used to limit the protection scope of the present invention.

[0054] In the following embodiments, the performance test of the wind power spindle steel is analyzed and tested in accordance with GB / T 228.1-2021. The metal microstructure is prepared conventionally and detected in accordance with the metal microstructure inspection method of GB / T 13298-2015. The X-ray diffraction method is used to irradiate the crystal surface of the sample to obtain the grain orientation information of the material.

[0055] Embodiment 1 A wind power spindle steel with low grain boundary energy and its preparation method

[0056] The preparation method of the wind power spindle steel with low grain boundary energy is as follows:

[0057] S1 Smelting:

[0058] S11 Add the raw materials into the furnace for smelting to make molten iron;

[0059] S12 Blow oxygen into the molten iron obtained in step S11 and add quicklime for slag-making treatment;

[0060] In S13, slag is removed to remove impurities, and molten steel for smelting is obtained. The elemental composition of the molten steel for smelting is by mass percentage: C: 0.04 - 0.06%; Si: 0.07 - 0.13%; Mn: 0.33 - 0.55%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and inevitable impurities;

[0061] S2 LF refining:

[0062] S21 Transfer the molten steel for smelting obtained in step S13 to an LF refining furnace;

[0063] S22 Heat up the molten steel in step S21, and add ferrovanadium and ferroniobium master alloys to adjust the chemical composition;

[0064] S23 Blow in argon for stirring, and then feed in aluminum wire for deoxidation;

[0065] S24 Supplement quicklime and fluorite, make slag, and obtain refined molten steel; the elemental composition of the refined molten steel is by mass percentage: C: 0.18 - 0.32%; Si: 0.21 - 0.38%; Mn: 1.11 - 1.34%; Nb: 0.41 - 0.49%; Cr: 1.82 - 1.96%; Mo: 0.36 - 0.41%; Al: ≤0.01%; V: 0.36 - 0.45%; Ti: 0.04 - 0.08%; N: 0.001 - 0.003%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and inevitable impurities;

[0066] S3 VD refining:

[0067] S31 Transfer the refined molten steel obtained in step S24 to a VD vacuum furnace, and conduct vacuum pumping treatment;

[0068] S32 Blow nitrogen from the bottom to break the vacuum and increase nitrogen in the molten steel;

[0069] S33 Feed in calcium wire to further purify the molten steel, and obtain high-nitrogen casting molten steel; the elemental composition of the high-nitrogen casting molten steel is by mass percentage: C: 0.18 - 0.32%; Si: 0.21 - 0.38%; Mn: 1.11 - 1.34%; Nb: 0.41 - 0.49%; Cr: 1.82 - 1.96%; Mo: 0.36 - 0.41%; Al: ≤0.01%; V: 0.36 - 0.45%; Ti: 0.04 - 0.08%; N: 0.010 - 0.014%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and inevitable impurities;

[0070] S4 Continuous casting and rolling:

[0071] S41 Pour the high-nitrogen cast steel obtained in step S33 into a mold for continuous casting to obtain a cast round billet;

[0072] S42 Perform blooming on the cast round billet obtained in step S41, then perform rolling. The rolling undergoes 2 deformations each of cross rolling and longitudinal rolling. The cast round billet is then hot delivered to an ingot at 900 °C to 950 °C;

[0073] S5 Profile forging:

[0074] S51 Heat the hot delivered ingot to 1260 °C to 1280 °C and hold for 5 h to 6 h to ensure uniform heating; then perform 2 times of repeated drawing and upsetting in sequence to obtain a repeatedly drawn and upset ingot;

[0075] S52 Reheat the repeatedly drawn and upset ingot from step S51 and then perform punching;

[0076] S53 Reheat the forging after punching in step S52 and then perform drawing and rolling of the shaft body to complete the forming of the shaft body and obtain rolled round steel;

[0077] S6 Performance heat treatment:

[0078] S61 Quickly place the rolled round steel obtained in step S53 into a slow cooling pit for multi-stage slow cooling treatment to obtain multi-stage slow cooled rolled round steel;

[0079] S62 Take out the multi-stage slow cooled rolled round steel from step S61, control the cooling to room temperature, and obtain wind power generator spindle steel.

[0080] Prepare 3 kinds of experimental wind power generator spindle steels according to this method. The process parameters and component parameters of each experimental wind power generator spindle steel are shown in Tables 1 - 3 respectively.

[0081] Table 1 Process parameters and component parameters of the wind power generator spindle steel in Experimental Example 1

[0082]

[0083]

[0084] Table 2 Process parameters and component parameters of the wind power generator spindle steel in Experimental Example 2

[0085]

[0086]

[0087] Table 3 Process parameters and component parameters of the wind power generator spindle steel in Experimental Example 3

[0088]

[0089]

[0090] Example 2 Analysis and Testing of Wind Power Spindle Steel

[0091] Prepare the following comparative examples:

[0092] Comparative Example 1: The difference from Experimental Example 1 is that in step S22, the addition amounts of ferrovanadium and ferroniobium master alloys are adjusted so that the Nb content in step S24, step S33, and the final product of wind power spindle steel is 0.52%, and the V content is 0.61%. Other steps and parameters are the same as those in Experimental Example 1.

[0093] Comparative Example 2: The difference from Experimental Example 1 is that the continuous casting and rolling process is changed to rolling with 3 deformations each by cross rolling and longitudinal rolling. The rolling temperature for the three cross rollings is 1000 °C, and the reduction rate is 3%; the rolling temperature for the three longitudinal rollings is 1050 °C, and the reduction rate is 5%.

[0094] Comparative Example 3: The difference from Experimental Example 1 is that the post-treatment process is changed to putting the rolled round steel into a slow cooling pit, covering it and sealing it, and cooling it to room temperature at a rate of 8 - 12 °C / s, instead of using multi-stage slow cooling.

[0095] Perform mechanical property tests, X-ray diffraction method detection, and metal microstructure observation on the samples of Experimental Examples 1 to 3 and Comparative Examples 1 - 3. The results are shown in Table 4 and Figure 1 、 2

[0096] Table 4 Test Results of Samples

[0097]

[0098] The tensile strength of the wind power spindle steel in Experimental Examples 1 - 3 exceeds 900 MPa, and the elongation rate exceeds 10%. The proportion of small-angle grain boundaries less than 10° in the wind power spindle steel of Experimental Examples 1 - 3 all exceeds 65%, reaching 67 - 69%. Micro-nano scale dispersed precipitates can be observed at the grain boundary positions of the wind power spindle steel in Experimental Examples 1 - 3, which are NbN precipitates and VN precipitates. See the typical metal microstructure pictures and partial enlarged views in Figure 1 。

Claims

1. A wind turbine main shaft steel, characterized in that: The element composition of the wind turbine main shaft steel is calculated by mass percentage as follows: C: 0.18-0.32%; Si: 0.21-0.38%; Mn: 1.11-1.34%; Nb: 0.41-0.49%; Cr: 1.82-1.96%; Mo: 0.36-0.41%; Al: ≤0.01%; V: 0.36-0.45%; Ti: 0.04-0.08%; N: 0.010-0.014%; P: ≤0.01%; S: ≤0.01%; the remainder is Fe and unavoidable impurities; the small-angle grain boundaries of less than 10° in the wind turbine main shaft steel account for 67%-69%; there is a micro-nano-sized (Nb, V)N precipitation phase at the grain boundary of the wind turbine main shaft steel; the preparation method of the wind turbine main shaft steel comprises the following steps: S1 Smelting: S11 adds raw materials into a furnace for smelting to produce molten iron; S12 blowing oxygen into the molten iron obtained in step S11, and adding quicklime for slag making; S13: removing impurities by slag blocking to obtain molten steel, wherein the element composition of the molten steel is as follows by mass percentage: C: 0.04-0.06%; Si: 0.07-0.13%; Mn: 0.33-0.55%; P: ≤0.01%; S: ≤0.01%; the remainder is Fe and unavoidable impurities; S2LF Refining: S21: transferring the molten steel obtained in step S13 to the LF refining furnace; S22: heating the molten steel in step S21 and adding ferrovanadium and ferroniobium master alloys to adjust the chemical composition; S23 blows in argon gas for stirring, and then feeds in aluminum wire for deoxidation; S24 adds quicklime and fluorite, makes slag, and obtains refined molten steel; the element composition of the refined molten steel is calculated by mass percentage as follows: C: 0.18-0.32%; Si: 0.21-0.38%; Mn: 1.11-1.34%; Nb: 0.41-0.49%; Cr: 1.82-1.96%; Mo: 0.36-0.41%; Al: ≤0.01%; V: 0.36-0.45%; Ti: 0.04-0.08%; N: 0.001-0.003%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and unavoidable impurities; S3VD Refining: S31: transferring the refined molten steel obtained in step S24 to a VD vacuum furnace and performing a vacuum treatment; S32 bottom blowing nitrogen to break vacuum and add nitrogen to molten steel; S33 feeds calcium wire to further purify the molten steel to obtain high nitrogen casting molten steel; the element composition of the high nitrogen casting molten steel is calculated by mass percentage: C: 0.18~0.32%; Si: 0.21~0.38%; Mn: 1.11~1.34%; Nb: 0.41~0.49%; Cr: 1.82~1.96%; Mo: 0.36~0.41%; Al: ≤0.01%; V: 0.36~0.45%; Ti: 0.04~0.08%; N: 0.010~0.014%; P: ≤0.01%; S: ≤0.01%; the balance is Fe and unavoidable impurities; S4 continuous casting and rolling: S41: pouring the high nitrogen casting molten steel obtained in step S33 into a crystallizer for continuous casting to obtain a casting round billet; S42: the cast round billet obtained in step S41 is subjected to a billeting process, and then rolled, wherein the rolling is deformed by two times of horizontal rolling and longitudinal rolling, and then the steel ingot is hot-sent at 900° C. to 950° C.; S5 Contour Forging: S51 The hot-delivered steel ingot is heated to 1260°C~1280°C and kept at this temperature for 5h~6h to ensure uniform heating; then, two repeated stretching-upsetting operations are performed in sequence to obtain a repeatedly stretched-upsetting steel ingot; S52: the steel ingot repeatedly stretched and upset in step S51 is returned to the furnace for heating and then punched; S53: After the forgings are punched in step S52, they are returned to the furnace for heating and then the shaft body is stretched and rounded to complete the shaft body forming to obtain round steel; S6 Performance Heat Treatment: Quickly place the round steel into the slow cooling pit and cover and seal it for 5h~8h for slow cooling. Then open the cover and blow air in for 15min~20min to form wind cooling. When the temperature drops to 300~400℃, cover and seal it for 8h~10h for slow cooling. Finally, take it out and place it in the air to cool naturally to room temperature.

2. The wind turbine main shaft steel according to claim 1, characterized in that: The smelting temperature in step S1 is 1600° C. to 1650° C., and the smelting time is based on the C content reaching the standard.

3. The wind turbine main shaft steel according to claim 1, characterized in that: The LF refining temperature in step S2 is 1720°C~1770°C, the argon flow rate is 200NL / min~300NL / min, the amount of aluminum wire fed is 4kg / ton~5kg / ton, the amount of quicklime supplemented is 8.6kg / ton~8.9kg / ton, the amount of fluorite supplemented is 1.8kg / ton~2.0kg / ton, and the refining time is calculated as 17min / ton~19min / ton.

4. The wind turbine main shaft steel according to claim 1, characterized in that: In step S3, the vacuum degree of VD refining is ≤55Pa, the refining temperature is 1780°C~1800°C, the nitrogen flow rate is 200NL / min~300NL / min, the feeding amount of calcium wire is 0.1kg / ton~0.12kg / ton, and the refining time is calculated as 16min / ton~19min / ton.

5. The wind turbine main shaft steel according to claim 1, characterized in that: In step S4, the first rolling temperature of the transverse rolling is 1150°C~1200°C, and the reduction rate is 15%~17%; the first rolling temperature of the longitudinal rolling is 1100°C~1150°C, and the reduction rate is 12%~14%; the second rolling temperature of the transverse rolling is 1050°C~1100°C, and the reduction rate is 10%~12%; the second rolling temperature of the longitudinal rolling is 1000°C~1050°C, and the reduction rate is 8%~10%.

6. The wind turbine main shaft steel according to claim 1, characterized in that: In the step S51, the hot-delivered steel ingot is heated to 1260°C~1280°C at a speed of 50°C / h~60°C / h, and kept warm for 5h~6h, followed by the first drawing and the first upsetting, with a drawing ratio of 7~8.5, an upsetting ratio of 7.3~7.6, and an end temperature of the first drawing-upsetting step of 1000~1050°C; followed by the second drawing and the second upsetting, with a drawing ratio of 3.8~4.2, an upsetting ratio of 3.8~4.1, and an end temperature of the second drawing-upsetting step of 750°C~780°C.

7. The wind turbine main shaft steel according to claim 1, characterized in that: The punched forging obtained in step S52 is returned to the furnace and heated to 1180°C~1200°C for shaft lengthening. The shaft lengthening is carried out according to the anvil width ratio of 0.5~0.7, the pressing amount is 80mm~100mm, and the feed amount is 0.4~0.6 of the anvil width. The shaft lengthening is carried out by turning 90° clockwise.

8. The wind turbine main shaft steel according to claim 1, characterized in that: The forging obtained in step S52 is subjected to rounding treatment at a temperature range of 750° C. to 960° C., with an anvil width ratio of 0.7 to 0.8 and a reduction of 8 mm to 16 mm.

9. Use of the wind turbine main shaft steel according to any one of claims 1 to 8 in the preparation of a wind turbine main shaft.

10. A wind turbine main shaft, characterized in that: The wind turbine main shaft is made of a wind turbine main shaft steel as described in any one of claims 1 to 8.

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